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Silicide formation through diffusion barriers

Balde, Maryna (2006-04)

Dissertation (PhD)--University of Stellenbosch, 2006.

Thesis

ENGLISH ABSTRACT: The formation of Ni-, Co- and Fe-silicides through different diffusion barrier interlayers was investigated. The diffusion barrier layers examined were Ta, Ti and Cr. In some cases the thickness of the barrier layer and the influence of a capping layer was also investigated. The thin-film structures were prepared on single crystal Si-substrates by Electron Beam Vacuum Deposition. The samples were vacuum annealed for times ranging from 10 to 60 min at temperatures ranging from 340 - 800°C and sample characterization was carried out by conventional RBS, dynamic RBS, channeling RBS and X-ray diffraction (XRD).
The use of a thin (20Å) Ta diffusion barrier in the Ni-Si system allowed no reaction even after annealing for 10 min at 400°C, but RBS measurements showed that after annealing for 15 min at 400°C uniform NiSi formed suddenly as first phase. XRD as well as dynamic RBS measurements confirmed this abrupt formation of NiSi instead of the normal first phase Ni2Si. According to the Effective Heat of Formation (EHF) model this shows that the diffusion barrier reduces the effective concentration of the Ni atoms to a value where the effective heat of formation of NiSi is more negative than that of Ni2Si and first phase formation of NiSi is thus thermodynamically favoured. The thickness uniformity of the first phase NiSi that formed through the thin Ta barrier improved at higher annealing temperatures. A thicker (100Å) Ta barrier also retarded the Ni diffusion and first phase, non-uniform NiSi only started to form at 500°C. The uniformity of this NiSi also improved with increased temperature but the use of the 20Å Ta barrier produced more uniform first phase NiSi in the 400 to 700°C temperature range. The use of a thin (30-50Å) Cr barrier also allowed the formation of mainly NiSi at 400°C, although XRD spectra indicated the presence of some Ni2Si. The uniformity of NiSi improved at higher temperature anneals. Similar results were obtained from samples with a thicker (100Å) Cr barrier layer at lower temperatures, i.e. the formation of NiSi as first phase at 400°C, but the first phase NiSi that formed at 500 to 700°C was non-uniform. In the case of Ti-barriers, the thicker (100Å) Ti barrier seems less effective than the thinner Ti barriers in delivering uniform first phase NiSi in the 500 to 700°C temperature range. The use of a thin (30-50Å) Ti barrier produced a mixture of Ni2Si and NiSi as first reaction at 400°C, but a 10 min anneal at 500°C formed uniform NiSi as confirmed by RBS and XRD measurements. The uniformity of the NiSi improved with an increase in annealing temperature up to 700°C. In the case of the thicker Ti interlayer no reaction occured at 400°C and non-uniform first phase NiSi formed at 500°C. All three thin barriers formed NiSi2 at temperatures of 750°C and above, but the thin Ti barrier formed the most uniform di-silicide. The NiSi2 that formed at 800°C through all three of the thicker barriers was non-uniform.
The use of a thin (10-30Å) Ta diffusion barrier prevented Co-silicide formation up to 560°C. The effective Co concentration at the growth interface is lowered, thus skipping the usual first phase formation of Co2Si at 450°C. At 560°C a mixture of CoSi and CoSi2 formed, as was confirmed by XRD. The CoSi2 that formed at 640°C (a higher formation temperature than without barrier) was of quite uniform thickness, but XRD measurements indicated that some CoSi was present as well. The use of thicker (100Å) Ta barrier layers retarded the diffusion of Co atoms for temperatures of up to 600°C. Annealing at 700°C formed CoSi2 and some CoSi and at 800°C non-uniform CoSi2 formed. The addition of a Ta capping layer (of different thicknesses) in conjunction with a 30Å Ta diffusion barrier layer did not significantly improve Co-silicide formation. The use of thin (10-30Å) Ti barrier layers resulted in the skipping of the Co2Si precursor phase and the formation of quite uniform first phase CoSi at 520°C. Uniform CoSi2 started forming at 560°C and the CoSi2 remained uniform at higher temperatures. The presence of a thicker (100Å) Ti barrier lowered the effective concentration of Co at the growth interface to such an extent that CoSi2 started to form as first phase after annealing for 30 min at 600°C. At 700 and 800°C non-uniform CoSi2 formed.
For Fe-silicide formation the use of 50Å and 100Å Cr barriers, as well as CrSi2 barriers, delivered very similar results. There was no change in the normal Fe-silicide phase formation sequence, as non-uniform FeSi was the first phase to form at 500°C and thereafter FeSi2 started to form at 600°C. At 700°C the use of Cr barriers resulted in the complete formation of FeSi2 of greater uniformity than was formed in the Si-Fe binary system without the presence of a diffusion barrier.
In this study dynamic real-time RBS has been used for the first time to prove without any doubt that diffusion barrier layers can be used to bring about “phase skipping”. These results have been interpreted in terms of the Effective Heat of Formation (EHF) model and are good examples of concentration controlled phase selection (CCPS). In general it was found that the thicker the diffusion barrier layer, the higher the temperature of silicide formation. Furthermore, silicide formation was generally found to be more uniform at higher annealing temperatures and when thinner diffusion barrier layers were used.